Conventionally, a navigation apparatus is so configured as to acquire information on the current location by means of a current location detector such as a GPS (global positioning system) receiver, then read map information recorded on a recording medium such as a CD-ROM or DVD-ROM, and then display a map around the current location on a display, while visibly or audibly giving guidance on the current location. When fed with a destination location, a navigation apparatus can also calculate a planned travel route from the current location to the destination location and guide through that route.
On the other hand, there have been developed, for car-mounted use and for incorporation in cellular phones, navigation apparatuses that receive map information, planned travel routes, and the like from a server apparatus over a network. Such a navigation apparatus does not require a recording medium having map information and the like recorded thereon, and has simply to receive information from a server apparatus whenever necessary. This helps realize a low-cost navigation apparatus. Moreover, incessantly updated map information and the like can be coped with simply by updating the information stored in the server apparatus.
A navigation system comprising a navigation apparatus and a server apparatus as described above can be realized, for example, in the form of one of the following three systems. Here, it is assumed that a certain amount of map information is stored in the navigation apparatus itself. The first system is one in which the server apparatus performs both the initial searching of a travel route and the searching of a travel route as performed on deviating from the initially planned travel route. The second system is one in which the navigation apparatus performs both the initial searching of a travel route and the searching of a travel route as performed on deviating from the initially planned travel route. The third system is one in which the server apparatus performs the initial searching of a travel route and the navigation apparatus performs the searching of a travel route as performed on deviating from initially planned travel route.
However, in the first system described above, the navigation apparatus does not always have the map information corresponding to the travel route found by the server apparatus. In such a case, the navigation apparatus needs to newly acquire the map information, and the map information that it currently has becomes useless. Also in the searching of a travel route as performed on deviating from initially planned travel route, the navigation apparatus needs to communicate with the server apparatus to acquire the results of searching. This results in a high communication fee.
In the second system, for the navigation apparatus to search for a travel route to a destination location, it needs to acquire map information covering a wide area from the current location to the destination location. This requires a large-capacity storage device, and thus makes such a system impracticable.
In the third system, as in the first system, where the server apparatus performs the initial searching of a travel route, the navigation apparatus does not always have the map information corresponding to the travel route found by the server apparatus. Moreover, the searching of a travel route as performed on deviating from the initially planned travel route needs to be performed within the area covered by the map information that the navigation apparatus currently has, and thus the result of searching tends to be a travel route that leads back to the initially planned travel route.
A navigation apparatus can receive and display map information with a scale factor that is previously set by the user. Here, every time the user wants to change the scale factor of the map that is displayed while the user is being guided through a travel route, the user is required to operate scale change keys or the like to obtain the desired scale factor.
For example, when the user, while being guided through a travel route, leaves a city area and enters an expressway, where detailed guidance on the travel route is no longer necessary, the user typically likes to switch to a map with a lower scale factor (i.e., with a larger coverage area). Thereafter, when the user exits the expressway and approaches the destination location, the user needs a detailed map again, and thus typically likes to switch back to a map with a higher scale factor.
However, manually changing the scale factor as described above is troublesome for the user who is driving. On the other hand, always receiving map information with a high scale factor without ever changing the scale factor involves a large amount of data, and thus results in a high communication fee.
Moreover, as to how much the user will be charged as a communication fee for the receipt of how much map information with what scale factor, the user has no choice but to make a rough calculation by himself. Thus, to save an unnecessary communication fee, the user needs to minimize the amount of received data by frequently changing the scale factor of the received map information so that it is received with as low a scale factor as possible.
As described above, the operation required for the user to perform to frequency change the scale factor of map information while being anxious about the communication fee is a severe burden on the user who is driving.